The above-identified Application discloses a stacker for stacking sheet materials such as, for example, sheets or blanks of paperboard for making corrugated containers. The blanks are conveyed on the upper surface of a plurality of side-by-side conveyor belts, and the belts are capable of being raised so that the discharge ends may be elevated relative to the inlet ends. The blanks are discharged from the discharge ends of the conveyor belts, after being reduced in velocity, and the blanks drop downwardly onto the stack located below the discharge ends of the conveyors. Preferably, a vertical, forward wall is provided such that the leading edges of the blanks abut the wall and thereby drop in alignment with the other blanks in the stack below.
This type of stacker is well-suited to the stacking of sheets or blanks which are relatively stiff, such as for example, unitary blanks of corrugated paperboard. That is, this type of stacker is well-suited to stacking blanks which are sufficiently rigid so as not to fold or crumple when they are ejected from the conveyor belts, or when they strike the forward wall while falling downwardly onto the stack below. However, many sheet materials are not sufficiently rigid to be handled in this manner. For example, thin paperboard or plastic sheets are not sufficiently rigid, and even relatively thick corrugated paperboard is not sufficiently rigid when it is in the form of an articulated sheet. As used herein, the term "articulated sheet" is intended to denote a sheet of material composed of a plurality of individual parts which are connected together by a plurality of small connecting portions as will be further explained hereinafter. These types of sheet material are too flexible to retain their planar configuration during discharge and stacking. Instead, they will fold upon themselves, or crumple, before they reach the stack below.
In addition to the problem of handling thin or articulated sheets, there is a serious problem of maintaining the stacking function at a speed consistent with that of the die-cutting machine which produces the blanks to be stacked. Such cutters may operate at speeds in the order of 1,000 feet per minute. This is a problem even if relatively tall stacks are to be formed, and it is a much greater problem when the stacks must be relatively short and each short stack must be moved away quickly from the stacking area while the next short stack is being formed. This problem is present in the case of articulated sheets where each stack must be relatively short so as to be able to be separated into individual portions in a downstream breaker as will be more fully described hereinafter.